Group iii{14 v semiconductor twinned crystals and their preparation by solution growth

ABSTRACT

The disclosure presents a method of growing large crystals of GaP from solution in which a zone of liquid gallium saturated with GaP is passed upward through a GaP feed ingot. In order to grow large crystals, it was found necessary to initiate growth onto a twinned seed crystal in which all of the twin planes are parallel to each other and parallel to a &lt;211&gt; growth direction. Wafers cut parallel to the twin planes exhibit a (111) surface which are suitable for electroluminescent devices.

United States Patent Blum et al.

[ 1 Feb. 15, 1972 [54] GROUP III-V SEMICONDUCTOR TWINNED CRYSTALS AND THEIR PREPARATION BY SOLUTION GROWTH [72] Inventors: Samuel E. Blum, New York; Luther M. Foaer, Chappaqua; Thomas S. I'laskett,

Ossining, all of NY.

[73] Assignee: International Business Machines Corporation, Armonk, NY. 7

[22] Filed: Aug. 19, 1968 211 Appl. No.: 753,522

[52] US. Cl. ..23l204 R, 23/301 R, 23/273 [5 1] Int. Cl. ..C0lb 25/08, Bold 9/00, BOIj 17/02 [58] Field ofSearch ..23/204 R, 30l R; l48/I.5,'l.6,

[56] References Cited UNITED STATES PATENTS 3,394,994 7/1968 Faust, Jr. et al. .....l48/l.6

OTHER PUBLICATIONS Broder et al.: A New Method of Gal Growth." Journal of the Electrochemical Society. Vol. H0, 1 [50-] I53 (1963).

Primary Examiner-Oscar R. Vertiz Assistant Examiner-Hoke S. Miller Attorney-Hanifin and Clark and Bernard N. Wiener [57] ABSTRACT 12 Claims, 8 Drawing Figures FURNACE POWER SUPPLY RADIO- FREQUENCY GENERATOR FURNACE POWER SUPPLY PATENTEBFEB 15 m2 3, 2,443

sum 1 OF 2 FIGJC FURNACE POWER SUPPLY RADIO- FREQUENCY GENERATOR FURNACE a POWER SUPPLY FIG. 1B

INVENTORS SAMUEL E. BLUM LUTHER M. FOSTER THOMAS S. PLASKETT ATTORNEY GROUP 'IIIi-V SEMICONDUCTORTWINNED CRYSTALS AND THEIR PREPARATION BY SOLUTION GROWTH BACKGROUND OF THEINVENTION Thisinventionrelates generally toa-method for growing twinned crystals and the crystals so grown, and it relates more particularly tov a. method of growing, twinned compound semiconductorcrystalsand the crystals'so grown.

Gallium phosphide is a potentially useful material for practical electroluminescent devices. The lack of suitable procedures for controlled bulkgrowth'of single crystal has deterred its exploitation therefor. Crystals of GaP have been prepared by precipitation from gallium or other metal solvent in which GaP- has an appreciable solubility. Usually, the crystals producedare thin dendritic platelets of various sizes and morphologies, i.'e., external structures.-lt is difficult, if not impossible, to form reproducibly-large and uniformly doped crystal platelets for device applicationsand physical characterization studies, However, a solution growth technique has" distinct advantages for the growth of Ga? and similar high melting and unstable compounds. Low-temperatures can be employed for off-stoichiometric growth, e.g., stoichiometric GaP melts at approximately 1,467 C. under approximately35 atmospheres of phosphorus pressure. The chief advantages of use of a solution growth'technique are the minimal equipment requirements..These advantages aredue to reduced pressure and limited contamination of the-growingcrystal from containcr materials. Another advantage of a solution growth technique is the purification-that results by dilution ofimpurities in the large volume-of solvent. For Ga as' a solvent, the solubility of GaPat. l,220 C. is approximately l percent by weight and the-P pressure is 0.02 atmospheres.-At temperatureof 1,220" C. several container materials are suitable, e. g., quartz, boron nitride,and-graphite.

Growth of a large bulk crystal ofgGaP bya solutiongrowth technique was described by .I. D. Broder et al., Journal of the Electrochemical Society, 110, l,l50 (1963). A highJemperature and saturated Ga-GaP zone .is passed through 'acomposite of liquid Ga .and solid GaP; Dissolution of GaP atthe leading edge of the zone and precipitation at the trailing edge of the zone produced a solid ingot with large grains.

Other semiconductor crystalmaterials have been fabricated in the prior art in sufficiently large sizes for practical devices.

However, thepurity thereof in both physical and chemical terms has been insufficient for desirable exploitation of the properties of the crystals. Illustratively, GaAs and IIIV alloy compounds such as Ga, .Al,As and'Ga ,As,P are semiconductor crystals which can properly utilize improved techniques'forpurification.

OBJECTS OF THE INVENTION It is anobject of this invention to provide a method for growing a twinned crystal by solution growth and the resulting product.

lt-is anotherv object of this invention to practice the foregoing .method-andprovide as theresulting product therefrom singletwinned crystals of lII.-V semiconductor material.

Itis anotherobject of thisinvention to provide a twinned Gal. crystal as'the semiconductor material of the preceding object.

It is another object of this -invention to provide a twinned seed crystalof GaP forrextended growth thereon of a twinned crystal of GaP by solution growth technique;

It is another objectof this invention to provide-an efficient procedure for solution growth of a twinned crystal-ofGaP on a twinned seed therefor.

It is another object of this invention to provide -a method of providing purified crystals.

It is another object of this invention to provide a method for purifying semiconductorcrystals.

It is another object of this invention to provide extended crystals by using solution growth on a twinned seed crystal.

SUMMARY OF THE INVENTION Generally, this invention provides a method for growing large twinned crystals by solution growth. The resulting crystals are purified with respect to the feed material and extended with respect to the seed.

Particularly, this invention provides for the controlled growth of crystalline GaP suitable for luminescent devices by selectivelygrowing an initial twinned crystal seed in a region at the expense of contiguous polycrystalline regions. Thereafter, bysolution growth, the twinned structure of the seed twinned crystal is propagated as extensively as is desired to grow a twinned GaP crystal. The-liquid zone for the solution growth of the crystalline GaP maybe Ga or other solvent in which GaP hasan appreciable solubility. However, the liquid BRIEF DESCRIPTIONOF THE DRAWINGS FIG. 1A is a schematic drawing'of the'apparatus for solution growth of twinned crystals by the practice of this invention.

FIG. 1B is an idealized drawing of a twinned-crystal'of'GaP grown according to the practice of this invention showing the nature ofthe twinned structure and'the growth direction.

FIG. ICis across section of the twinned crystal of FIG. 1B taken parallel to the twin planes showing the nature of the crystalline orientation.

FIG. 2 illustrates the'steps of the procedure of'providing a twinned seed crystal in the practice of this invention.

FIGS. 2A, 2B, 2C and 2D are sectional views illustrating the surface morphology of the solid-liquid interface of thegrowing crystal shown in Steps'l,2, 4 and 6, respectively, of FIG. 2.-

DESCRIPTION OF THE DRAWINGS The structure and'operation of the apparatus 10'of FIG. for growing a twinned crystal by the practice of this invention through solution growth will now be described. 'A quartz tube .l8supports the boron-nitride crucible 16. The seal and suportion .28-1, a 'lower'furn'ace portion '28-2, andsingl'e-turn radiofrequency coil 20. The furnace portions 28-1 and 28 2 have ceramic furnace shells 30-1 and 302, respectively. Resistance windings 29 1 and 29-2 are placed vertically in shells 30-1 and 30-2, respectively, to'limitthe inductive coupling from the 'radiofreque'ncy coil 20. The upper and lower furnaces 28-1 and 28-2 are controlled separately by constant voltage powersupplies 34-1 and 34-2. For control purpose. alternating current powersupplies are used. The single-turn radiofrequencycoil 20 is energized by radiofrequency generator 24. I

A layer or zone 12 ofGa approximately 0.25-inch thick .is

-pla'ced'between the seed 14 and polycrystalline'feed'ingot 15 of GaP, all of which are contained in pryolytic boron-nitride crucible .16. The loaded crucible I6 is sealed under'vacuum or backfilled with an inert or doping gas in quartz tube '18 with a minimum of free space between the outer wall of crucible l6 and the -'.inner wall of quartz tube I8The zone 12 of Ga is approximately l,l-60 C. by single-turn and 22 from 'radiofrequency generator 24. At temperature-of approximately 1,160 C., the zone 12 dissolves about to per- .cent'of its weight of GaP. Initially, the material-is dissolved both'from the seed l4 and from feed ingot l6 surfaces 26 and 27, respectively. The Ga zone 12 temperature may conveniently be estimated by its GaP content as determined after completion of growth of a GaP crystal and with the use of solubility data from the literature.

The upper and lower furnaces 28-1 and 282, shown as extending above and below the radiofrequency coil 20, control the shape of the solid-liquid interface 26. Furnace 28-1 is energized via wires 31-1 and 32-l from constant voltage power supply 341. Furnace 28-2 is energized via wires 31-2 and 32-2 from constant voltage power supply 342. The quartz tube 18 is rotated in the radiofrequency coil 20 and furnaces 281 and 28-2 as indicated by arrow 35 at approximately 8 r.p.m. to smooth out radial temperature nonuniformities. The entire quartz tube 18 and 6a? preparation material therein are passed through furnace 28 in the direction of arrow 28A by a drive 27A attached to quartz tube 18 to achieve relative motion to the furnace; The mechanical mo tion is obtained in a conventional manner, not shown. The extension 18A of sealed quartz tube 18 is also used to maintain precise axial movement of crucible 16 in furnace 281 and 28-2, and radiofrequency coil 20.

It was discovered for the practice of this invention that an essentially nonporous feed ingot 15 is desirably employed for the successful passage of the liquid zone 12. Otherwise, a void accumulates above the gallium zone 12 and eventually isolates it from the feed ingot. Completely sound single-phase polycrystalline ingots 15 were synthesized from Ga and PH in a vertical open system using the same size crucible as is used here. This synthesis is described in copending application (IBM Docket No. Y09-68-026) filed July 11,1968, by T. S. Plaskett. and assigned to the assignee hereof. The synthesis was carried out in a boron-nitride crucible of the same dimensions as crucible 16'of FIG. 1A. Other procedures for providing the feed ingot 15 may be used, e.g., the procedures described by S. E. Blum et al. in the Journal of the Electrochemical Society, Vol. 1 I5, 1968, page 324 and the Journal ofrhe Electrochemical Society, Vol. 1 15, 1968, page 298.

The structure of the twinned regrown crystal 14 is one in which the twinned planes are parallel to each other and parallel to 211 growth direction. This is the structure of Ga? crystal platelets randomly nucleated from solution. As a result of the twins, the growing solid-liquid interface consists of indestructible grooves, which provide favorable sites for nucleation as shown in FIG. 1B. It is believed that the same growth mechanism applies to the spontaneous growth-of dendritic GaP platelets from dilute gallium solutions.

FIG. 1B presents a perspective view of a model of an idealized twin crystal. The actual twin planes seldom propagate so perfectly. Usually, they wander by jogs or terminate and new lamellar parallel twin planes are initiated. This is evident from inspection ofa microphotograph, not shown, ofa cross section and a longitudinal section of the regrown crystal. For example, FIG. 1C is a schematic representation ofa typical longitudinal sectional view of the crystalline structure of FIG. 1B taken parallel to the twin planes. FIG. 1C shows that there are two crystalline regions. The faces exposed on both are parallel to 111 faces. The contiguous crystals are oriented such that the 1 10 directions in the (111) plane are separated by 60. It has been demonstrated heretofore that the desirable growth plane for electroluminescent GaP devices is the (111) plane. The effective nature of the plane of FIG. 1C is that both regions constitute a (11 1) plane. The surface parallel to the twin planes in general constitutes a (111) plane whatever may be the number of contiguous crystals.

Twinning in GaP does not detract from its usefulness for electroluminescent devices. The twin planes intersect at lowenergy coherent. boundaries that generally do not perturb transportof electrical charge carriers across them. Further, a known method of: forming the PN-junction in Ca? for the productionof light emitting diodes employs a solution overgrowth technique onto (11]) oriented substrates. The only faces exposed when wafers are cut parallel to the twin planes are the (111) faces.

The steps in the procedure for obtaining a twinned seed crystal for growth of a twinned crystal according to the practice of this invention is illustrated in FIG. 2. In FIG. 2 the crucible 16 is shown removed from the furnace 10 environment of FIG. 1A for clarity of exposition, it being understood that in practice the crystal growth is achieved in the furnace 10 environment. Initially, acrystalline section achim ed by a suitable prior art growth procedure for a Grip crystal. eg. the procedures described by S. E. Blum et al. in the noted arti cles in the Journal of the Electrochemical Society. is prepared of the requisite size of the inner dimension of the crucible 16. The lower portion 14A as illustrated in FIG. 1A supports the crystalline region 100 at the boundary 102 therebetween. Actually, the boundary 102 disappears during evolution of crystalline growth on the substrate portion 100 via the solution growth in the apparatus of FIG. 1A. The initial seed structure 100 has a twinned region 104 parallel to the growth axis 28A. The remainder portion 108 thereof is a large grained and nontwinned polycrystalline structure. For the steps of the procedure illustrated in FIG. 2 in growing a seed twinned crystal, the seed ingot 110 should be of high purity and nonporous as described above with reference to the growth of twinned crystal 14 in the apparatus of FIG. 1A. The step 2 of FIG. 2 shows the evolution of crystal growth on the seed 100 indicating that the twinned portion 104 grows preferentially at the expense of the polycrystalline region 108 thereby achieving a sizable twinned crystalline portion 112. In step 3, the uppermost portion 114 of the grown crystal of step 2 is indicated as the starting material for the growth procedure of step 3. Again, the same section 14A is retained as the supporting structure for the section 114. The procedures of steps 1, 2, and 3 of FIG. 2 are continued respectively through steps 4 and 5 until finally in some step, e.g., step 6, the entire upper portion of the evolved crystalline structure 122 has become completely a twinned crystal which is then utilized as the support seed crystal for-the procedure illustrated in FIG. 1A and described herein.

FIGS. 2A, 2B, 2C and 2D are sectional views illustrating the surface morphology of the solid-liquid interface of the growing crystal shown in Steps 1,2, 4 and 6, respectively, of FIG. 2.

TEMPERATURE GRADIENT One essential condition for the growth of good twinned crystals is e.g., shallow temperature gradient across the growing solid-liquid interface. The gradient can be varied by changing the power to the top and bottom furnaces 281 and 28-2 and the radiofrequency power to the zone 12. A number of experiments were conducted with various power settings until the optimum morphology was achieved. Typical settings for twinned crystal growth are described below. The radiofrequency power to the zone 12 was adjusted to give a temperature of l,l60 C. this temperature was determined by analyzing the Ga? concentration in the zone and then referring to published liquidus temperatures, e.g., the article by C. D. Thurmond, Journal ofPhysics and Chemistry ofSolids, Vol. 26, Pergammon Press, l965,pages 785-802. The top and bottom furnaces 281 and 28-2 were adjusted to read temperatures of 986 C. and 714 C. on thermocouples placed near to the furnace windings and about /2 inch above and below the radiofrequency coil 20. Increasing the gradient by increasing the zone temperature resulted in polycrystalline growth.

Another feature for good growth of a twinned crystal is that the shape of the growing solid-liquid interface 26 should be convex to the liquid. If it is concave, stray nucleation on the walls of the crucible 16 propagate into the center of the twinned crystal. This was controlled by a tine adjustment of the lower furnace 29-2 to reduce radial heat losses and thus to insure an uniaxial heat flow pattern. The low temperature gradient and the convex solid-liquid interface requirements are partially related, i.e., a change in one produced a change in the other.

The morphology of the crystal was found to be greatly dependent upon the impurity concentration of the feed ingot 15. Apparently, high purity N-type feed ingots give much better crystals than high purity Ptype crystals. Silicon doped feed ingots, to provide N-type crystals at doping levels of about atoms/co, gave the best doped crystals. The dopants and impurities influence the thermodynamic properties of the growing interface.

EXPERIMENTS FOR THE INVENTION At an exemplary growth rate of approximately 4 mm./day, and with the axial temperature gradient as described previously across the zone 12, the zone travelled through the polycrystalline ingot with no visible entrapment of the Ga solvent. An idealized crystal grown by this technique is shown in P16. 18. The growing interface is slightly convex to prevent polycrystalline growth around the periphery of the crystal.

The original seed crystal was a large grained ingot with a very small twinned grain of the preferred morphology. Regardless of size and orientation of the grains in the seed crystal, the twinned structure propagated through the ingot l5 and consisted of lamellar twins whose growth direction 21 l was parallel to the growth direction 28A. After approximately 6 cm. of crystal growth, the twinned structure had developed completely at the expense of the nontwinned structure. The resultant twinned seed crystal 14 was used subsequently to grow several twinned crystals of the desired morphology in both doped and undoped forms.

The chemical and electrical properties of a GaP crystal grown according to the practice of this invention will now be discussed. Table 1 shows data on emission spectroscopic analysis of a P-type starting feed ingot l5 and of the grown twinned crystal after one solution zone pass and of the subsequent twinned crystal after two solution zone passes.

TABLE L-Emission Spectrochemical Analysis (p.p.m. by wt.)

Starting 1st pass 2d pass Element Bottom Top Bottom Top Bottom Top 10 10 10 10 10 1-3 3-10 3-10 3 3 0.3 0.3 0.3-1. 0 0. 3-1. 0 0. 3 3-10 3 3 N .D. 3 3 3 3 3 3 1 N.D. N.D. N.D. N.D. N.D. N.D. 3 N.D. N.D.

N OTE.-N D Not determined.

The entries in Table l are order of magnitude figures; the term "N.D." means not detected. When less than a given concentration is indicated, a line was observed on the emission spectrochemical photographic plate but adequate calibration was not available to provide narrower sensitivity limits. The analyses show a reduction in the silicon and iron content with additional solution zone passes. Therefore, the method of this invention produced a twinned crystal of higher purity than that of the starting feed ingot. Therefore, it has been demonstrated that the purity of a resultant twinned crystal can be controlled by control of the number of solution zone passes and the volume of the zone.

Table 11 shows Hall data for another P-type twinned crystal resulting from two zone passes. Included for comparison in Table 11 are data for N-type twinned crystal resulting from one zone pass. After each zone pass, the resulting twinned crystal is used as the feed ingot for the next zone pass.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof. it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

We claim:

1. Method of growing a lll-V semiconductor crystalline material comprising the steps of:

establishing a seed crystal ofa lll-V semiconductor material with a twinned structure in a crystal solution growth environment, said solution growth environment including a liquid zone adjacent said seed crystal at a liquid-solid growth interface and a nonporous feed ingot of said lll-V semiconductor material adjacent said liquid zone at a solid-liquid interface, said liquid zone having substantial solubility for said feed ingot at one temperature and insubstantial solubility in said seed crystal at a lower temperature; and

propagating said twinned structure preferentially at said solid-liquid interface in a shallow temperature gradient across said solid-liquid interface in the direction of crystal growth.

2. Method as set forth in claim 1 wherein said liquid-solid growth interface is convex tosaid liquid zone.

3. Method as set forth in claim 1 wherein said lIl-V semiconductor is GaP.

4. Method as set forth in claim 3 wherein said liquid zone is Ga. 1

5. Method according to claim 3 wherein said twinned crystal structure has twin (111) planes parallel to the 21 l direction and grows in said 211 direction, and sections parallel to said twin planes exhibit multiple crystalline regions with 110 directions respectively with contiguous regions being oriented such that the 1 10 directions in said regions respectively are separated by 60.

6. Method of purifying a III-V semiconductor crystalline material comprising the steps of:

establishing a twinned crystalline material in a crystal solution growth environment having a liquid for said solution growth at a first solid-liquid interface with said seed crystal; establishing a nonporous feed ingot of said crystalline material at a second solid-liquid interface with said liquid;

said liquid having substantial solubility for said feed ingot at one temperature and insubstantial solubility in said seed crystal at a lower temperature; and

purifying said crystalline material by at least one solution zone pass by propagating said twinned structure preferentially at said first solid-liquid interface in a shallow temperature gradient across said first solid-liquid interface in the direction of crystal growth.

7. Method as set forth in claim 6 wherein the number of said solution zone passes is aplurality.

8. Method as set forth in claim 6 wherein said crystalline material is Gal.

9. Method of producing a twinned seed crystal of a lll-V semiconductor material comprising the steps of:

establishing a crystalline structure of said l1lV semiconductor material in a solution growth environment at a solid-liquid interface, said crystalline structure having a twinned region parallel to the growth direction in said solution growth environment; and propagating said twinned structure preferentially over any adjacent crystalline structure at said solid-liquid interface TABLE II.ELECTRICAL P ROPE RTIES Hall Carrier Hall Carrier Resistivity, mobility, concentra- Resistivity, mobility, cpncentror Twinned crystal description ohm-cm. emJ/tn-see. tion, cmr ohm-cm. cmfl/m-sec. tion, CHI-'3 A two solution zone pass growth through a p-type feed ingot 1 0. 29 2.2X10 1.04 960 6.3)(10 15 A one solution zone pass growth through an n-type feed ingot... 2 0. 26 142 1.6Xl0 232 340 7.8)(10 1 (t pe) 2 (n-type) in a shallow temperature gradient across said solid-liquid interface in said growth direction. 10. Method as set forth in claim 9 wherein said twinned crystalline material is GaP.

11. A llI-V semiconductor bulk crystalline structure with a twinned crystal morphology and with an extended size substantially greater than that of a dendritic platelet.

12. A crystalline structure as set forth in claim 11 wherein 

2. Method as set forth in claim 1 wherein said liquid-solid growth interface is convex to said liquid zone.
 3. Method as set forth in claim 1 wherein said III-V semiconductor is GaP.
 4. Method as set forth in claim 3 wherein said liquid zone is Ga.
 5. Method according to claim 3 wherein said twinned crystal structure has twin (111) planes parallel to the <211> direction and grows in said <211> direction, and sections parallel to said twin planes exhibit multiple crystalline regions with <110> directions respectively with contiguous regions being oriented such that the <110> directions in said regions respectively are separated by 60* .
 6. Method of purifying a III-V semiconductor crystalline material comprising the steps of: establishing a twinned crystalline material in a crystal solution growth environment having a liquid for said solution growth at a first solid-liquid interface with said seed crystal; establishing a nonporous feed ingot of said crystalline material at a second solid-liquid interface with said liquid; said liquid having substantial solubility for said feed ingot at one temperature and insubstantial solubility in said seed crystal at a lower temperature; and purifying said crystalline material by at least one solution zone pass by propagating said twinned structure preferentially at said first solid-liquid interface in a shallow temperature gradient across said first solid-liquid interface in the direction of crystal growth.
 7. Method as set forth in claim 6 wherein the number of said solution zone passes is a plUrality.
 8. Method as set forth in claim 6 wherein said crystalline material is GaP.
 9. Method of producing a twinned seed crystal of a III-V semiconductor material comprising the steps of: establishing a crystalline structure of said III-V semiconductor material in a solution growth environment at a solid-liquid interface, said crystalline structure having a twinned region parallel to the growth direction in said solution growth environment; and propagating said twinned structure preferentially over any adjacent crystalline structure at said solid-liquid interface in a shallow temperature gradient across said solid-liquid interface in said growth direction.
 10. Method as set forth in claim 9 wherein said twinned crystalline material is GaP.
 11. A III-V semiconductor bulk crystalline structure with a twinned crystal morphology and with an extended size substantially greater than that of a dendritic platelet.
 12. A crystalline structure as set forth in claim 11 wherein said III-V semiconductor is GaP and wherein said twinned crystal morphology has (111) twin planes and <211> crystal direction, said (111) twin planes of said twinned crystal morphology being parallel to said <211> crystal direction and sections therein parallel to said twin planes exhibit multiple crystalline regions having <110> directions respectively with contiguous regions being oriented such that said <110> directions in said regions respectively are separated by 60* . 